ASSESSMENT OF THE FATE OF 2 HERBICIDES IN A WYOMING RANGELAND SOIL - COLUMN STUDIES

Extensive use of dicamba (2-methoxy-3,6-dichlorobenzoic acid) and picl oram (4-amino-3,5,6-trichloropicolinic acid) in arid Wyoming, along wi th large volumes of irrigation water used in some areas, has created a concern for the potential contamination of surface and groundwaters b y these herbicides. Persistence and mobility of dicamba and picloram w ere investigated in a Wyoming rangeland soil using batch adsorption an d soil column studies. The objectives of this study were to characteri ze sod chemical and physical properties that affect herbicide transpor t, examine herbicide sorption, model herbicide movement, and estimate degradation rate constants. Essentially no sorption of dicamba was det ected; however, picloram sorption was greatest in the highest organic C content horizon. Both saturated (5.90, 2.%, and 0.82 kg ha-1 dicamba and 1.85, 0.97, and 0.47 kg ha-1 picloram) and unsaturated (2.76 and 1.00 kg ha-1 for dicamba and picloram, respectively) column experiment s were conducted. The herbicides and Br tracer (34, 38, 69, and 137 mg L-1) were displaced through the soil columns using distilled water th at was added in daily increments (60 mL d-1). Degradation rate constan ts were calculated using both a simple recovery fraction technique and by matching LEACHP-generated breakthrough curves to experimental data . For the two columns receiving intermediate application rates, anaero bic picloram dissipation was more rapid (t1/2 = 19 d) than for aerobic conditions (t1/2 = 87 d). The rate of dissipation of dicamba was appr oximately the same under aerobic and anaerobic conditions (t1/2 = 15 a nd 17 d in the saturated and unsaturated columns, respectively). Piclo ram and dicamba dissipation was more rapid at the lowest application r ates, with t1/2 of 13 and 10 d. At the highest application rates, t1/2 of 23 and 17 d were measured for picloram and dicamba, respectively. Both herbicides were found to be highly mobile, with the mobility of p icloram increasing at higher pore-water velocities.